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Environmental Hydraulics and Sustainable Water Management – Lee & Lam (eds) © 2005 Taylor & Francis Group, London, ISBN 04 1536 546 5 Preliminary numerical study on groundwater system at the Hong Kong International Airport Fuli Wang Lincoln Ventures Ltd, Lincoln, Christchurch, New Zealand Jiu J. Jiao Department of Earth Sciences, University of Hong Kong, Hong Kong, China ABSTRACT: The Hong Kong International Airport was built on a man-made island. The ground- water head of the deep basal sand and gravel aquifer remains high several years after the land reclamation. The heads in different aquifers indicate an upward groundwater flow. The aim of this study is to understand the abnormally high pore water pressure in the basal aquifer. The most plausible explanation for the abnormal pressure distribution is that the groundwater flow below the airport is probably a part of the regional groundwater flow system of the Lantau Island. The airport is located at the upward discharge zone of the Lantau groundwater flow system. Based on groundwater level measurement in the airport site, a multi-aquifer model is used to simulate groundwater flow in the regional flow system. Simulation results illustrate that the high pore pres- sure in the basal aquifer can be generated by the regional flow system in the Lantau Island and the pore pressure is strongly affected by the seasonal variation in rainfall recharge and the hydraulic conductivity of the alluvial clay. 1 INTRODUCTION Large-scale land reclamation may introduce substantial changes to original geological, hydro- geological and environmental conditions. Understanding such changes is important for properly designing and maintaining engineering projects on reclaimed land. The Hong Kong new airport was built on a largely man-made island. Several years after the reclamation, piezometers in the deep basal sand and gravel aquifer show that the groundwater head of the confined alluvial aquifer remains much higher than that in the unconfined aquifer consisting of fill materials, suggesting a significant upward flow. The aim of this study is to understand the abnormally high pore water pressure in the basal aquifer and the upward flow in the aquifer system. A plausible explanation for the abnormal pressure distribution is that the groundwater flow below the airport is probably a part of the regional groundwater system of the Lantau Island, which has a catchment area of much larger than the airport. The airport is located at the upward discharge zone of the Lantau groundwater system. In this paper, conceptual and mathematical models are estab- lished to describe groundwater flow in the multi-aquifer system below the airport using available borehole data and groundwater piezometric measurements. Then numerical simulations are used to investigate the major factors affecting groundwater flow dynamics. 2 BASIC GEOLOGY AND HYDROGEOLOGY OF THE AIRPORT The Airport is located to the north of Lantau Island (Figure 1). Its total area is about 12.5 km2,of which 75% was reclaimed from the sea. The remainder comprises the former islands of Chek Lap Kok and the Lam Chau. Detailed geologic information about the airport can be found in Plant, et al. 2047 (1998). A simplified geological section through A–B is shown in Figure 2. Bedrock consists of igneous intrusive and volcanic rocks in various states of weathering. It is believed that the fractured rock zone along the rockhead may be very permeable (Jiao and Nandy, 2001). Such a relatively high permeability (K) zone is referred to as HKZ and is treated as a fractured aquifer in this study. Figure 1. Model domain (plane view) and rainfall recharge zones. Figure 2. Geological cross section along A–B (see Figure 1 for its location) (not to scale). 2048 The completely decomposed rocks (CDR) are less permeable and are treated as semi-confining unit for the underlying HKZ. Next in the sequences is Chek Lap Kok Formation alluvium. The lower part of this formation consists mainly of the basal sand and gravel and the upper part consists mainly of clay. The alluvial sand and gravel become a confined aquifer. The alluvial clay above the sands provides a semi-permeable unit. The top layer in the natural sequence was predominantly very soft to soft marine muds. The mud was dredged prior to reclamation and replaced by fill materials, which consist of marine sands and rock boulders. The fill materials become a very good unconfined aquifer (Jiao and Tang, 1999). The average annual rainfall is 2225 mm. The mean sea level is at 1.25 mPD. Observations of groundwater head show a significant head difference between the alluvial and the fill aquifers. The average head in 18 piezometers in the fill is only 1.51 m. However the average head in 31 piezometers in the alluvial clay and sand is 3.32 m, 1.81 m higher than that in the fill. The model domain is divided into five zones (Figure 1). Zone 1 has six layers: marine mud, alluvial clay, alluvial sand and gravel, CDR, HKZ, and much less permeable bedrock. The sea is the top boundary of Zone 1. Similarly, Zone 2 also consists of six layers, with the fill material as the topmost layer. Zone 3 has four layers: alluvial sand, CDR, HKZ and bedrock. Zone 4 has only three layers: CDR, HKZ and bedrock. Zone 5 is located at the top of hills of Lantau Island where HKZ is exposed to land surface and can be recharged by rainfall infiltration. 3 MATHEMATICAL AND NUMERICAL MODELS The top boundary is the land surface with rainfall recharge or the seabed treated as a specified head boundary (Figure 1). The no-flow bottom boundary is set somewhere in the bedrock below the HKZ. All lateral boundaries are treated as no-flow boundaries: the south boundary is the groundwater divide in the Lantau Island; the east and west boundaries are approximately along the flow lines; and to the north, the system is buried progressively deep and lateral flow may be negligible. The governing equation for water flow under variably saturated conditions is: where K is the hydraulic conductivity (L/T), h is the pressure head (L), q is source or sink term (L3/T), z is the vertical coordinate (L), t is time (T), and F is the storage coefficient expressed as: where θ is moisture content, ne is porosity, α and β are the modified compressibilities of media and water, respectively. The rainfall recharge coefficient is assigned based on the hydraulic property and types of the top soil (Table 1). Table 1. Rainfall recharge coefficient. Land surface material Recharge coefficient (%) Rock 0.3 CDR 0.2 Alluvial sand and gravel 0.5 Fill under vegetation 0.4 Fill under building 0.15 Fill under runway 0.1 2049 FEMWATER, a three-dimensional finite-element model (Lin, et al., 1997), is chosen as the numer- ical code for solving the mathematical model. Each geologic formation is divided into at least three numerical layers. Thus the aquifer system is represented by 18 numerical layers. Each numerical layer is discretised into 1661 triangular elements. 4 NUMERICAL SIMULATION Model calibration was conducted by comparing simulated groundwater heads with measured ones. Due to limited data, only hydraulic conductivities are calibrated. The target of model calibration is to minimize the difference between the average observed heads and the average simulated heads at 18 piezometers in the fill aquifer and 31 piezometers in the alluvial aquifer. Comparisons between observed and simulated heads are shown in Figure 3. The model parameters are shown at Table 2. It is evident that observed heads show larger variations than simulated ones especially in the alluvial sand aquifer, but the difference between observed and simulated average heads is reasonably small. The large difference between observed and simulated heads is due to the fact that the simulated heads are steady-state solutions and observed ones are heads measured at specific times, and that the spatial variation in the thickness and hydraulic conductivity of alluvial clay and sand layers was not well presented in the model. 3 2.5 2 1.5 Total head (mPD) 1 0.5 (A) 0 0 2 4 6 8 10 12 14 16 18 20 7 Observed Simulated Average of observed Average of simulated 6 5 4 3 Total head (mPD) 2 1 (B) 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 IDs of observational points Figure 3. Comparison between observed and simulated heads in the fill (A) and alluvial (B) aquifers. 2050 Table 2. Values of model parameters. Parameter Symbol (Unit) Value Source Conductivity of fill aquifer K1 (m/day) 172.8 Calibrated Conductivity of marine mud K2 (m/day) 0.001 Calibrated Conductivity of alluvial clay K3 (m/day) 0.002 Calibrated Conductivity of alluvial aquifer K4 (m/day) 43.2 Calibrated Conductivity of CDR K5 (m/day) 0.001 Calibrated Conductivity of HKZ K6 (m/day) 2.16 Calibrated Conductivity of bed rock K7 (m/day) 0.000864 See note Compressibility of water α (m d2/kg) 5.89 × 10−20 See note 2 −18 Compressibility of sand β1 (m d /kg) 2.68 × 10 See note 2 −18 Compressibility of clay β2 (m d /kg) 5.36 × 10 See note 2 −20 Compressibility of rock β3 (m d /kg) 1.34 × 10 See note Note: A very low value was given to K7 to represent the impermeable bottom; the values of the last 4 parameters were taken from typical values from the FEMWATER website (Reeve, 1998); Water retention curves for all layers were chosen from the GMS database. For speeding up model convergence, unsaturated conductivities were assumed to be linear functions of moisture content.

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